* feat(select): improve merchant dropdown behavior and placement controls
- add configurable menu_placement strategy to DS::Select (auto/down/up) with safe normalization
forward menu_placement through StyledFormBuilder#collection_select
- force Merchant dropdown to open downward in transaction create and editor forms
- fix select option/search text contrast by applying text-primary in DS select menu
- prevent form jump on open by scrolling only inside dropdown content instead of using scrollIntoView
- clamp internal dropdown scroll to valid bounds for stability
- refactor select controller placement logic for readability (placementMode, clamp) without changing behavior
* set menu_placement=auto for metchant selector
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